Power Boost/Buildup mechanics
Basically, that.
The important point is that, in the guts of the system, resistance effects are like damage effects with negative strength (I'm sure Arcanaville will be along shortly with the details). As such, they're affected by things which modify damage effects - such as damage buffs. In other words, if you had a brute running a resistance shield, as soon as fury started ramping up that shield would be providing an extra 200% of its base resistance values, aka doubling its enhanced value. If you had 45% before, now you have 90%.
Obviously that's a problem, and the devs have two ways to stop that from happening. The first is to take the resistance granting buff in the power, and mark it as 'ignores enhancements and buffs'. You can see this on FF's deflection shield. The other way is to take the entire power and flag it as 'not affected by outside buffs/debuffs', as in cold dom's ice shield.
Both prevent the resistance effect from being erroneously boosted by damage buffs on the character. The first, since it doesn't flag the entire power, still allows power boost to function. However, this is only ok for powers where the resistance component isn't intended to be enhanceable. For powers which it is, either because the resistance is the primary purpose or just because the devs felt like it, the whole power has to be flagged to ignore outside buffs/debuffs. That stops the problem while still allowing enhancement, but only at the cost of preventing it from being affected by things like power boost.
So, rule of thumb, any power with an enhanceable resistance component can't be affected by power boost, because if it was able to be affected, then the resistance component would be able to be boosted by damage buffs.
@MuonNeutrino
Student, Gamer, Altaholic, and future Astronomer.

This is what it means to be a tank!
Beaten to the punch!
This works both ways, though. In regards to FF vs. Cold, a FF user hit by a -Special power like Weaken would have the +Defense values lowered on any shields they cast after being hit. But a Cold user's shields would not be affected by -Special because of the "ignore outside buff/debuffs" flag.
Edit: Also, the reason this issue only ever comes up in regards to +Resistance is because all +Damage, -Damage, and -Resistance effects are flagged "Ignores Enhancements & Buffs", anyway.
Can anyone clarify the exact reason that PB affects bubbles but not ice shields?
I vaguely remember a mechanics explanation from years ago... Something about not being able to affect defense powers with resist components without screwing some other thing up? I've been going over City of Data and I can't figure out what the problem would be.